Personnel in and on moving vehicles can be subjected to high acceleration environments, such as a crash or quick velocity change that give rise to rapid relative motion between the vehicle and the occupant. These environments create a risk of injury due to contact with vehicle or equipment structures, and a risk of inadvertent ejection from the vehicle. The risk of injury is especially pronounced with personnel who cannot be restrained in a typical seat restraint system due to mobility requirements, such as mobile aircrew members within the cargo bays of military helicopters.
Systems currently in use for restraining helicopter mobile aircrew include the gunner's belt with fixed-length tether, or a vehicle-mounted tether and reel system. The drawbacks to the gunner's belt with a fixed-length tether are that the user is free to flail at any point within the sphere of reach of the tether; some tethers are not adjustable in length; and some tethers must be continually adjusted manually by the user, interrupting critical mission duties.
Fixed-length tether systems allow unwanted free travel of the user at high velocities relative to the vehicle, unwanted contact with the vehicle structures at high velocity, and possible inadvertent ejection or partial ejection from the vehicle due to unwanted free travel. This gives rise to contact injuries, acceleration injuries and compression injuries, as well as the dangers associated with travel outside the vehicle envelope.
The vehicle-mounted tether and reel system addresses some of the drawbacks of the fixed-length tether system by using a retractable reel that takes up slack in the tether between the user and the reel. A reel locking mechanism, which is based on sensed acceleration, can lock the reel to prevent the user or crew member from further travel in the event of high acceleration event. A drawback to the vehicle-mounted tether and reel system is that the reel locking mechanism is based on acceleration of the tether unwinding from the reel, which leads to inadvertent locking of the reel during high vibration dynamics or in turbulent wind conditions, when there is very little relative motion between the user and the vehicle. They also require attachment to the vehicle at a point capable of supporting a 5000-pound load. There are few vehicles that can meet this requirement without major structural changes. The structural changes are cost prohibitive and prevent the installation of the potentially life saving technology. The vehicle-mounted systems are not compatible with attachment at existing D-ring locations that crewmembers currently use for attaching fixed-length tethers.